Effects Of Different Discharge And Slope Gradient On Erosion Mechanism Of Colluvial Deposits In Granite Collapsing Hill

Granite collapsing hill is widely distributed in the tropical and subtropical South China.The colluvial deposits as an important part of granite collapsing hill,feature high contents of gravel,sand,and loose materials that have a weak structure,low cohesion,poor stability,and high erodibility.The erosion of colluvial deposits can affect the redistribution of materials in the collapsing walls/slopes and thereby impact the degree of gully damage.However,the erosion process and mechanisms of colluvial deposition have rarely been reported.The test soil is the granite colluvial deposits collected from Yangkeng Village,Longmen town,Anxi County,Fujian Province.This study aimed to analyze the processes of sediment yield,the rill erosion characteristics and the dynamic parameters under different discharge and slope gradient conditions.Discharge ranged from 3.6-14.4 L/min and slope gradient from 20-40°.The main conclusions are as follows:(1)The processes of sediment yield in colluvial deposits can be divided into three stages:increasing gradually,fluctuation and stable;The changes of sediment concentration with time have smooth and multimodal types.Discharge has greater effect than slope gradient on sediment yield,while it has smaller influence than slope gradient on sediment concentration.With the increase of sediment concentration,Sediment yield increases with increasing sediment concentration with a linear function.Discharge and slope gradient cab be used to predict sediment yield and sediment concentration.(2)The place of the rill erosion occurs at the upper part of the slope.When the discharge and slope gradient increase,the time that the rill erosion happened is shorten and the width-depth radio decreases.With the discharge and slope gradient increase,the main slope erosion pattern changes from sheet erosion to rill erosion and the contribution of rill erosion to slope sediment yield also increases.(3)The percentage contents of coarse sand and fine silt are higher than others.Except the gravel particle,the other particle enrichment ratios(ER)are higher than 1.When the discharge and slope gradient increase,the mean weight diameter(MWD)of rill erosion sediment yield increases;And with the discharge and slope gradient increase,the contents of suspention load and saltation load reduced before they are stable,and the content of bed load increases before it is stable.The average contents of suspention load and saltation load are between 68.45%and 81.39%,and the average content of bed load is between 18.61%and 31.55%.(4)The flow velocity and the flow depth increase with the discharge and slope gradient increase,and the effects of discharge is greater than slope gradient on flow velocity and flow depth.The relationships between the flow velocity/flow depth with the discharge/slope gradient show a power function.The relationships between the sediment yield rate of rill erosion and flow velocity,the flow depth,Reynolds number and Fred number represent extremely significant or significant positive correlation,and the flow velocity shows the most impact.The shear flow,unit stream power and specific energy in sections increase with the discharge and slope gradient increase,and discharge has greater effect than slope gradient on them.Theirs critical values are 5.706Pa,0.044J/s and 0.148cm,respectively.The unit stream power can be the primary choices to be the dynamic parameters of colluvial deposits' sediment transportation under discharge conditions.